TL;DR

As noted earlier, transducers can be properly annotated in Clojure using core.typed and they probably should be.

But... there are a few tricks necessary to make it work.

Transducers in Scala require tricks too, but different ones.

Oh, but they're so lovely in Haskell.

Update 2015-01-12

Were you led here by Clojure Gazette? Eric Normand is usually more discriminating, but don't worry, this
will only waste a little of your time. Per the previous batch of updates, just below, and various subsequent
posts on more or less the same topic, it should be clear this wee bagatelle …